In this chapter, the committee summarizes the findings of previous National Research Council (NRC) committees as they examined the question of the safety of water reuse. Building on the risk assessment methodologies presented in Chapter 6, the committee then presents a comparative analysis of potential health risks of potable reuse in the context of the risks of the common contemporary circumstance of a conventional drinking water supply derived from a surface water that receives a small percentage of treated wastewater (see Chapter 2). By means of this analysis, the committee compares the estimated risks of a drinking water source generally perceived as safe (i.e., de facto potable reuse) against the estimated risks of two other potable reuse scenarios.

The 1982 Committee on Quality Criteria for Water Reuse, citing the many unknowns in making an assessment of the health effects of potable reuse, adopted the view that “the quality of reused water could be compared to that of conventional drinking water supplies, which are assumed to be safe” (NRC, 1982). That committee was providing advice for an extensive testing program being undertaken by the U.S. Army Corps of Engineers on the treatment of an effluent-dominated Potomac River as part of an evaluation of the future water supply for Washington, D.C., and it outlined specific testing procedures for evaluating the treated water based on the state of science in 1982. A second NRC committee reviewed the results of the Corps’ testing program and found them inconclusive, primarily because the Corps had not included all the tiers of toxicological testing recommended (NRC, 1984).

In the years that followed, more extensive toxicological tests were conducted for other proposed potable reuse projects, particularly in Tampa (CH2M Hill, 1993) and Denver (Lauer and Rogers, 1996). In reviewing those data (see Box 6-2), which did not demonstrate any adverse health effects, a third NRC committee concluded that such tests provide a database too limited to draw general conclusions about the safety of potable reuse (NRC, 1998). NRC (1998) also pointed out new concerns that had arisen since the earlier NRC reports and outlined new testing techniques that had been developed. The committee also recommended that new toxicity tests be conducted, particularly long-term fish exposure testing, which were partially implemented in subsequent evaluations in Orange County and Singapore. Advice on testing methods will continue to evolve as science advances, but based on the progression of this research, it is evident that, although such testing might be used to show evidence of potential health risk, it cannot be used to establish the absence of risk.

Examining this history from the vantage point of 2011, the most profound contribution of the 1982 committee was the idea that the quality of the water in potable reuse scenarios should be compared with the quality of conventional drinking waters, which are assumed safe. Advice on specific tests that might be useful in making these comparisons will continue to follow developments in the underlying science, but it is unlikely that any laboratory test will ever establish the absence of health risk in a drinking water from any source. This chapter builds on that foundation,

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7
Evaluating the Risks of Potable Reuse in Context
In this chapter, the committee summarizes the ily because the Corps had not included all the tiers of
findings of previous National Research Council (NRC) toxicological testing recommended (NRC, 1984).
committees as they examined the question of the In the years that followed, more extensive toxi-
safety of water reuse. Building on the risk assessment cological tests were conducted for other proposed
methodologies presented in Chapter 6, the committee potable reuse projects, particularly in Tampa (CH2M
then presents a comparative analysis of potential health Hill, 1993) and Denver (Lauer and Rogers, 1996). In
risks of potable reuse in the context of the risks of the reviewing those data (see Box 6-2), which did not dem-
common contemporary circumstance of a conventional onstrate any adverse health effects, a third NRC com-
drinking water supply derived from a surface water that mittee concluded that such tests provide a database too
receives a small percentage of treated wastewater (see limited to draw general conclusions about the safety of
Chapter 2). By means of this analysis, the committee potable reuse (NRC, 1998). NRC (1998) also pointed
compares the estimated risks of a drinking water source out new concerns that had arisen since the earlier NRC
generally perceived as safe (i.e., de facto potable reuse) reports and outlined new testing techniques that had
against the estimated risks of two other potable reuse been developed. The committee also recommended
scenarios. t hat new toxicity tests be conducted, particularly
long-term fish exposure testing, which were partially
implemented in subsequent evaluations in Orange
PREVIOUS NRC ASSESSMENTS
County and Singapore. Advice on testing methods
OF REUSE RISKS
will continue to evolve as science advances, but based
The 1982 Committee on Quality Criteria for on the progression of this research, it is evident that,
Water Reuse, citing the many unknowns in making an although such testing might be used to show evidence
assessment of the health effects of potable reuse, ad- of potential health risk, it cannot be used to establish
opted the view that “the quality of reused water could be the absence of risk.
compared to that of conventional drinking water sup- Examining this history from the vantage point
plies, which are assumed to be safe” (NRC, 1982). That of 2011, the most profound contribution of the 1982
committee was providing advice for an extensive testing committee was the idea that the quality of the water
program being undertaken by the U.S. Army Corps of in potable reuse scenarios should be compared with
Engineers on the treatment of an effluent-dominated the quality of conventional drinking waters, which are
Potomac River as part of an evaluation of the future assumed safe. Advice on specific tests that might be
water supply for Washington, D.C., and it outlined useful in making these comparisons will continue to
specific testing procedures for evaluating the treated follow developments in the underlying science, but it
water based on the state of science in 1982. A second is unlikely that any laboratory test will ever establish
NRC committee reviewed the results of the Corps’ the absence of health risk in a drinking water from
testing program and found them inconclusive, primar- any source. This chapter builds on that foundation,
123

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124 WATER REUSE
comparing the estimated health risk of water from potable reuse projects with the risk associated with
potable reuse projects with conventional supplies using de facto reuse scenarios that are representative of the
established tools of risk assessment. supplies that are widely experienced today. The com-
The 1982 committee went on to say that the com- mittee chose to construct a “risk exemplar” to examine
parison should be made with the highest quality water how these comparisons might be made. The analysis
that can be obtained from that locality even if that in this exemplar uses the quantitative risk assessment
source may not be in use. In a similar vein, the 1998 methods originally proposed for organic chemicals by
committee, after concluding that planned potable reuse the NRC (1983) as expanded for microbial contami-
is viable, suggested that planned potable reuse should nants (Haas et al., 1999) and more recently updated
be, “an option of last resort—to be adopted only if all (NRC, 2009b). Other methods recently developed to
the alternatives are technically or economically infea- address pharmaceuticals, personal care products, and
sible” (NRC, 1998). All three committees (NRC, 1982, other anthropogenic contaminants (Rodriquez et al.,
1984, and 1998) also took the view that U.S. drinking 2007a,b; Snyder et al., 2008a; Bull et al., 2011) are
water regulations were not intended to protect public also used in the analysis to address the risk of classes
health when raw water supplies were heavily contami- of contaminants for which rigorous toxicological data
nated with municipal and industrial wastewater. are lacking. In the committee’s judgment, these risk
In the committee’s judgment, current circum - assessment techniques represent the best means avail-
stances call for a reassessment of those views. First, as able at this time for estimating the relative risk in such
shown in Chapter 1, the United States has been operat- circumstances (see Chapter 6) and offer a method for
ing near the limit of its water supply for several decades evaluating the relative merits of various options for
since about the time of the first study. As a result of managing health risks from chemical and microbial
further stress from continued population growth and contaminants in reclaimed water.
climate change, this report is being written with a view The committee chose to develop an exemplary
to providing useful advice to the nation as it comes to comparison of risks associated with various potable
terms with this new world where pristine water is ever reuse scenarios, including de facto potable reuse, mod-
less abundant, even as the domestic wastewater from eled upon circumstances currently encountered in the
an increasing population is discharged into the nation’s United States today. Based on the discussion in Chapter
waterways. Second, as demonstrated in Chapter 2, the 2, the committee concluded that it would be appropri-
committee concludes that de facto reuse (i.e., when ate to compare the quality of the water in potable reuse
a drinking water source consists of some significant scenarios with the quality of a de facto reuse scenario
percentage of treated wastewater effluent from an up- where a conventional water supply has an average an-
stream discharger) is becoming increasingly common nual wastewater content of 5 percent. This situation is
in the United States. Finally, it has become evident commonly found among current surface water supplies
to the committee that, in many communities, today’s (see Box 2-4). As shown in the figure in Box 2-4, there
drinking water regulations are already being employed are many circumstances where de facto reuse exceeds
to address the quality of drinking water prepared from 5 percent, and the committee discussed at length the
water supplies that have substantial wastewater content appropriate wastewater content for use in the exem-
(see also Chapter 10 for a discussion of regulations). plar. In the end, 5 percent was selected as a wastewater
Although this fact does not imply that the regulations content that can be reasonably viewed as commonplace
are adequate for that charge, it does reflect a notable and not exaggerated. Swayne et al. (1980) reported that
shift in perspectives since the prior NRC reuse reports more than 24 million people of the 76 million people
were written. surveyed were using drinking water supplies with a
wastewater content of 5 percent or more in low-flow
conditions (see figure in Box 2-4). Although no data ex-
THE RISK EXEMPLAR
ist, anecdotal evidence based on the population growth
Under these conditions, the committee judges that in urban areas suggests that wastewater content is often
it is appropriate to compare the risk associated with higher today.

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EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT
The comparative risk approach used in this analy- percent contribution of disinfected wastewater efflu-
sis was designed to examine the presence of selected ent upstream of the intake for the drinking water plant
pathogens and trace organic chemicals in final product (Figure 7-1a). The two reuse cases describing drinking
waters from de facto reuse and two potable reuse sce- water supplies derived from planned potable reuse proj-
narios. Contaminant occurrence data, compiled from ects include one with groundwater recharge to a potable
several sources, were critically evaluated for each sce- aquifer via surface spreading basins with subsequent
nario. The data were then analyzed to assess whether soil aquifer treatment (SAT; Scenario 2, Figure 7-1b)
there are likely to be significantly greater human health and one with groundwater recharge to a potable aquifer
concerns from exposure to contaminants in these hypo- by direct injection of reclaimed water that has received
thetical reuse scenarios, compared with a common de advanced water treatment (Scenario 3, Figure 7-1c).
facto reuse scenario. For the chemicals in each of the
scenarios, a risk-based action level was used, such as Scenario 1: De Facto Reuse (Common Surface Water
the U.S. Environmental Protection Agency’s maximum Supply)
contaminant levels (MCLs), Australian drinking water
guidelines, or World Health Organization drinking In Scenario 1, a surface water supply that serves as
water guideline values. Also, a margin of safety is re- a drinking water source receives discharge from second-
ported, defined as the ratio between a risk-based action ary treated wastewater effluent that is disinfected and
level (such as an MCL) and the actual concentration dechlorinated prior to discharge to meet a standard of
of a chemical in reclaimed water. The resulting ratio 200 fecal coliform/100 mL. The surface water is as-
between these two values (i.e., the margin of safety) can sumed to be free of pathogens with no measurable trace
be used to characterize potential health risks associated organic chemicals prior to effluent discharge.
with exposure to a chemical (Illing, 2006). For mi- Attenuation of contaminants after wastewater
crobials, the dose-response relationships were used to discharge can vary widely as a function of distance
compute risk from a single day of exposure. Additional between discharge point and raw drinking water with-
underlying assumptions are described below. drawal (i.e., retention time), streamflow geometry (i.e.,
It was beyond the means of the committee to depth, mixing), and environmental conditions such as
conduct an analysis of every possible contaminant in temperature, ultraviolet light penetration, particulate
reclaimed water. In addition, certain assumptions were matter, biological activity. In this scenario, a worst
made to simplify the analysis. The committee focused case is assumed where no inactivation or attenuation
on four pathogens commonly of concern in reuse appli- of pathogens or chemicals occurs in the surface water
cations and selected 24 chemicals representing different body. The wastewater discharged constitutes 5 percent
classes of contaminants (i.e., nitrosamines, disinfection of the flow in the source at the point where water is
byproducts (DBPs), hormones, pharmaceuticals, anti- abstracted for the drinking water treatment plant.
microbials, flame retardants, and perfluorochemicals), Subsequently, this water is extracted by a conventional
for which occurrence and toxicological data were avail- drinking water plant employing coagulation/floccula-
able in the published literature. tion, followed by granular media filtration with disin-
fection designed to meet the requirements of the Long
Term-2 Surface Water Treatment Rule (EPA, 2006a).
Potable Reuse Scenarios Considered in the
Exemplar
Scenario 2: Soil Aquifer Treatment
Three hypothetical scenarios were evaluated to
compare the relative risk from exposure to pathogens In Scenario 2, a potable aquifer is augmented with
and trace organic chemicals in the conventional water reclaimed water via groundwater recharge by surface
supply and potable reuse scenarios. Scenario 1, the spreading. Advanced wastewater treatment in this
conventional water supply scenario, considers de facto exemplar assumes secondary treatment, followed by
reuse with a conventional drinking water treatment nitrification, partial denitrification and granular media
plant drawing water from a supply that receives a 5 filtration but no disinfection. The reclaimed water is

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126 WATER REUSE
FIGURE 7-1 Summary of scenarios examined in the risk exemplar. (a) Scenario 1—A conventional water plant drawing from a
source that is 5 percent treated wastewater in origin; (b) Scenario 2—A deep well in an aquifer fed by reclaimed water via a soil
aquifer treatment system and (c) Scenario 3—A deep well drawing from an aquifer fed by injection of reclaimed water from an
advanced water treatment (AWT) plant.
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Figure 7-1

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EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT
applied to surface spreading basins with subsequent mg-min/L requirement in California’s Title 22 regu-
SAT (see Chapter 4). It is assumed that the water lations was considered, but was not included because
remains in the subsurface for 6 months with no dilu- sufficient data could not be obtained to estimate the
tion from native groundwater. The assumption of no impact of disinfection on contaminant concentrations.
dilution is a worst case, being more conservative than From a qualitative perspective, this scenario would re-
the real-world hydrogeological characteristics of typi- sult in significantly lower levels of microbial exposure,
cal subsurface systems. This condition was selected to particularly for Salmonella, but higher levels of DBPs
assign removal credits only to physicochemical and would be present—trihalomethanes and haloacetic ac-
biological attenuation processes occurring during SAT. ids if free chlorine is used or N-nitrosodimethylamine
Subsequently, the water is abstracted and disinfected (NDMA) if combined chlorine is used. A review of the
with chlorine at the wellhead prior to consumption, literature shows that these DBPs are typically removed
assuming that no blending with other source waters during the SAT, particularly NDMA (Kaplan and
occurs in the distribution system. This assumption de- Kaplan, 1985; Yang et al., 2005; LACSD, 2008; Zhou
scribes a scenario where all the drinking water that is et al., 2009; Nalinakumari et al., 2010; Patterson et al.,
consumed originates from the reclaimed water source. 2011), but vigilance is called for when disinfected efflu-
This assumption is conservative, given that most exist- ent is used for SAT because as shown in the following
ing potable reuse projects blend their product water section, the margin of safety is smaller with DBPs than
with other sources, providing additional dilution. with most other trace organic chemicals.
Further details for Scenarios 1 through 3 are pro-
vided in Appendix A with respect to water quality char-
Scenario 3: Microfiltration, Reverse Osmosis, Advanced
acteristics, attenuation, and generation of contaminants
Oxidation, Groundwater Recharge
during various treatment steps.
In Scenario 3, a potable aquifer is augmented with
advanced-treated wastewater followed by groundwater Contaminants Considered in the Risk Exemplar
recharge by direct injection. The advanced treatment
train includes secondary treatment with chloramina- The committee considered a broad cross section
tion, followed by microfiltration, reverse osmosis, and of common pathogenic bacteria, viruses, and protozoa,
advanced oxidation using UV irradiation in combina- as well as regulated and nonregulated trace organic
tion with hydrogen peroxide (UV/H2O2). It is assumed chemicals that have been reported in reclaimed water
that the water remains in the subsurface for 6 months or surface water receiving wastewater discharge, to
with no dilution from native groundwater. Again, this determine the contaminants to be considered in the
scenario assumes that any attenuation of pathogens and risk exemplar. The contaminants that were ultimately
trace organic chemicals in the aquifer is achieved only selected met the following criteria: (1) sufficient infor-
by physicochemical and biological processes rather than mation was available on their occurrence, health effects,
dilution. Subsequently, the groundwater is abstracted fate and transport, and behavior in treatment systems
and disinfected at the wellhead with chlorine prior such that reasonable calculations could be made for
to consumption. Again, this case describes a scenario each scenario; and (2) they are either recognized to be
where 100 percent of the drinking water consumed of concern based on possible health effects or they are
originates from reclaimed water after advanced water of interest to the public.
treatment and direct injection into a potable aquifer. Four pathogens were selected: adenovirus, norovi-
This assumption is also conservative, given that most rus, Salmonella, and Cryptosporidium. All of these or-
existing potable reuse projects blend their product ganisms are transmitted by the fecal-to-oral route, and
water with other sources, providing additional dilution. they all play an important role in waterborne illness in
the United States. All four pathogens have been stud-
ied in effluents, and, for each, dose-response data are
Other Scenarios Considered
available. Salmonella is a classic bacterial pathogen as-
The construction of an additional scenario, similar sociated with both food- and waterborne disease, while
to Scenario 2, but with disinfection similar to the 450 the significance of the other three pathogens has only

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128 WATER REUSE
Assumptions Concerning Fate, Transport,
become clear in recent decades. Toxigenic Escherichia
Removal, and Estimates of Risk
coli was originally considered as well, but sufficient
dose-response data were not available.
The assumptions in the exemplar concerning the
Potential adverse health effects associated with
fate, transport, and removal of the pathogens and
trace organic chemicals in drinking water are an im-
chemicals considered in each scenario of the exemplar
portant concern among stakeholders and the public.
are discussed in detail in Appendix A. Literature ref-
As noted in Chapter 3, reclaimed water can contain
erences are provided for the sources of the data that
chemicals originating from consumer products (e.g.,
make up each scenario, including expected densities or
personal care products, pharmaceuticals), human waste
concentrations following the various treatment steps
(e.g., natural hormones), and industrial and commercial
(including both engineered treatment systems and
discharges (e.g., solvents). The reclaimed water, itself,
engineered natural systems), to characterize the fate
can also contain compounds that are generated during
of contaminants from the initial water or wastewater
water treatment (e.g., DBPs). Collectively, the number
source to the product water at the consumer’s tap.
of potential compounds present in reclaimed water is
Quantitative microbial risk assessment methodologies
in the thousands. For the risk exemplar, 24 chemicals
are described that are used to estimate the risk of dis-
were selected (see Box 7-1) that represent different
ease that results. Chemical risk assessment techniques
classes of these contaminants (i.e., DBPs, including
used in the exemplar are also described that detail
nitrosamines; hormones; pharmaceuticals; antimicrobi-
methods to derive risk-based action levels for chemicals
als; flame retardants; and perfluorochemicals).
in reclaimed water.
The starting concentrations for the microbial and
chemical contaminants were selected on the basis of a
Exemplar Results
review of contaminant occurrence data in the scientific
literature. More details are provided in Appendix A.
The goal of the exemplar exercise is to illustrate
the relative risk among the scenarios. In Appendix A,
the qualities of the surface and reclaimed water and the
final water qualities are described for each case as well
BOX 7-1
Chemicals Selected for Evaluation in the as the rest of the assumptions behind the exemplar.
Risk Exemplar Scenario 1 represents a scenario to which the public is
already commonly exposed in many locations through-
Hormones and
out the United States and which is generally regarded
Disinfection Byproducts Pharmaceuticals
as safe, whereas Scenarios 2 and 3 represent planned
17β-Estradiol
Bromate
potable reuse projects. Because of the nature of the risk
Bromoform Acetaminophen
characterization tools employed, risks from pathogens
Chloroform (paracetamol)
Dibromoacetic acid (DBCA) Ibuprofen are displayed in a different form than the risks from
Dibromoacetonitrile (DBAN) Caffeine
chemicals. The pathogen risks are calculated as an esti-
Dibromochloromethane (DBCM) Carbamazepine
mate of the risk of increased gastroenteric illness. These
Dichloroacetic acid (DCAA) Gemfibrozil
Dichloroacetonitrile (DCAN) Sulfamethoxazole data also can be usefully displayed as a relative risk—the
Haloacetic acid (HAA5) Meprobamate
risks of the potable reuse Scenarios 2 and 3 relative to
Trihalomethanes (THMs) Primidone
the risks of Scenario 1—de facto reuse (Figure 7-2).
N-Nitrosodimethylamine Others
Figure 7-2 presents a summary of the relative
(NDMA)
Triclosan
risk of illness from exposure to norovirus, adenovirus,
Tris(2-chloroethyl)phosphate
Salmonella, and Cryptosporidium as a result of drink-
(TCEP)
ing water from each of the three scenarios. All of the
Perfluorooctanesulfonic acid
(PFOS) risks have been normalized to Scenario 1, the de facto
Perfluorooctanoic acid
potable reuse example. As shown, both potable reuse
(PFOA)
scenarios have reduced risks, especially where viruses

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EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT
particular process trains produce water of an acceptable
quality. Note that these assessments were based on an
ingestion scenario. For other end uses, such as shower-
ing, some modification in the analyses would need to
be made.
For the pharmaceuticals, triclosan, and TCEP,
the margin of safety ranges from 1000 to 1,000,000
for all three scenarios (a margin of safety lower than 1
poses potential concern). The perfluorinated chemicals
(PFOA and PFOS) have lower margins of safety, but
have margins of safety exceeding 1 for all three sce-
narios. With one exception, the DBPs are shown to
have margins of safety above 1. For NDMA, the data
for all three scenarios show that it was below the limit
of detection, but the detection limit (2 ng/L) exceeds
the 10–6 lifetime cancer risk level used for the risk-
based action level for this compound in the exemplar
(0.7 ng/L). As a result the margin of safety for NDMA
can only be established as greater than 0.35 for all
FIGURE 7-2 Relative risk of illness (gastroenteritis) to persons
three scenarios. These results have not identified any
drinking water from each of the reuse scenarios relative to de
chemical that presents a health risk of concern in any
facto reuse (Scenario 1). The smaller the number, the lower the
relative risk of the reuse applications for each organism. For of the scenarios studied, although further research is
example in Scenario 2, the risk of illness due to Salmonella is
warranted to ensure confidence in these assessments
estimated to be less than 1/100th of the risk due to Salmonella
(see Chapter 11). Despite uncertainties inherent in
in Scenario 1.
NOTES: *The risks for Salmonella and Cryptosporidium in the analysis, these results demonstrate that following
R02129
Scenario 3 were below the limits that could be assessed by the
proper diligence and employing appropriately designed
Figure 7-2
model.
treatment trains (see Chapter 5), potable reuse systems
bitmapped
can provide protection from trace organic contaminants
comparable to what the public experiences in many
drinking water supplies today. As a general rule, DBPs
are concerned with the SAT supply, and with all four
and perfluorinated chemicals deserve continued scru-
organisms where the microfiltration/reverse osmosis/
tiny in all drinking water supplies.
UV supply is concerned. In the latter instance, the
For microbial agents, if one illness or infec-
densities of Salmonella and Cryptosporidium are esti-
tion/10,000 persons per year is used as a benchmark, it
mated to be reduced to such low levels that the model
is apparent that the risks from bacterial and protozoan
was unable to calculate a risk. On the basis of these
exposure are below this benchmark for all the scenarios,
calculations the committee concludes that microbial
with the exception of Scenario 1, the de facto reuse ex-
risks from these potable reuse scenarios are much less
ample (see Appendix A, Table A-6). In this particular
than those from de facto reuse.
instance, it is likely that the risks for the viruses are
Table 7-1 summarizes the estimates of the margin
overestimated, perhaps as a result of the conversion of
of safety for each of the 24 organic compounds studies
the genome copy density to the density of infectious
in the exemplar. These are also displayed graphically
units (IU) and/or because predation and die-off in the
in Figure 7-3.
stream was neglected. In any case, the consistent use of
conservative assumptions throughout all three scenarios
Findings of the Risk Exemplar
assures that the assessment of the relative risk of one
scenario over the other is robust. The relative analysis
The results of the risk assessment (Table 7-1, Fig-
makes it clear that the potable reuse scenarios exam-
ures 7-2 and 7-3) can be used to ascertain whether the

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130 WATER REUSE
TABLE 7-1 Summary of Margin of Safety (MOS) Estimates for the Three Scenarios Analyzed by the Committee
Risk-Based MOS Scenario 1, MOS Scenario 2, MOS Scenario 3
Chemical Action Levela de Facto Reuse SAT, No Disinfection MF/RO/UV
Nitrosamines
>0.4 >0.4 >0.4
NDMA 0.7 ng/L
Disinfection byproducts
>2
Bromate 10 µg/L N/A N/A
>160
Bromoform 80 µg/L 27 160
Chloroform 80 µg/L 16 80 16
>60 >60 >60
DBCA 60 µg/L
>54 >140
DBAN 70 µg/L N/A
>80 >160
DBCM 80 µg/L N/A
>60 >60
DCAA 60 µg/L 12
>20 >20
DCAN 20 µg/L N/A
HAA5 60 µg/L 6 12 12
THM 80 µg/L 2.7 16 8
Pharmaceuticals
>350,000,000 >350,000,000 >35,000,000
Acetaminophen 350,000,000 ng/L
>120,000,000 >280,000,000
Ibuprofen 120,000,000 ng/L 56,000,000
>190,000,000
Carbamazepine 186,900,000 ng/L 10,000,000 1,200,000
>140,000,000
Gemfibrozil 140,000,000 ng/L 8,600,000 2,300,000
>80,000,000 >160,000,000
Sulfamethoxazole 160,000,000 ng/L 720,000
>930,000,000
Meprobamate 280,000,000 ng/L 17,000,000 8,800,000
>58,000,000
Primidone 58,100,000 ng/L 10,000,000 450,000
Others
>70,000,000 >23,000,000
Caffeine 70,000,000 ng/L 3,500,000
17-β Estradiol >35,000,000 >35,000,000 >35,000,000
3,500,000 ng/L
>3,500,000 >2,100,000
Triclosan 2,100,000 ng/L 840,000
>84,000 >210,000
TCEP 2,100,000 ng/L 5,800
>200
PFOS 200 ng/L 17 4
>80
PFOA 400 ng/L 36 19
NOTES: > indicates that the assumed concentration was below detection, and only an upper limit on the risk calculation was determined. See Appendix A
for further detail. aSources of the risk-based action limits are provided in Table A-11 of Appendix 11.
ined here represent a reduction in microbial risk when CONCLUSIONS
compared with the de facto scenario that has become a
It is appropriate to compare the risk from water
common occurrence throughout the country.
produced by potable reuse projects with the risk as-
It should be emphasized that the committee pres-
sociated with the water supplies that are presently in
ents these calculations as an exemplar. This should
use. The committee conducted an original compara-
not be used to endorse certain treatment schemes
tive analysis of potential health risks of potable reuse
or determine the risk at any particular site without
in the context of the risks of a conventional drinking
site-specific analyses. For example, the presence of a
water supply derived from a surface water that receives
chemical manufacturing facility in the service area of a
a small percentage of treated wastewater. By means of
wastewater utility being used for potable reuse would
this analysis, termed a risk exemplar, the committee
dictate scrutiny of chemicals that might be discharged
compared the estimated risks of a common drinking
to the sanitary sewer. In addition, the various inputs
water source generally perceived as safe (i.e., de facto
and assumptions of this risk assessment contain sources
potable reuse) against the estimated risks of two other
of variability and uncertainty. Good practice in risk
potable reuse scenarios.
assessment would require full consideration of these
The results of the committee’s exemplar risk
factors, such as by a Monte Carlo analysis (Burmaster
assessments suggest that the risk from 24 selected
and Anderson, 1994).
chemical contaminants in the two potable reuse sce-

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EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT
FIGURE 7-3 Display of the Margin of Safety (MOS) calculations for the 24 chemicals analyzed for each of the three scenarios. MOS
<1 is considered a potential concern for human health.
NOTE: Bars with diagonal stripes are for MOS values represent the lower limit of the actual value, considering that the concentration
of the contaminant was below the detection limit. R02129
Figure 7-3
bitmapped degree of uncertainty, the committee’s analysis
narios does not exceed the risk in common existing great
water supplies. The results are helpful in providing suggests the risk from potable reuse does not appear
to be any higher, and may be orders of magnitude
perspective on the relative importance of different
lower than currently experienced in at least some
groups of chemicals in drinking water. For example,
current (and approved) drinking water treatment
DBPs, in particular NDMA, and perfluorinated chemi-
systems (i.e., de facto reuse). State-of-the-art water
cals deserve special attention in water reuse projects be-
cause they represent a more serious human health risk treatment trains for potable reuse should be adequate
than do pharmaceuticals and personal care products, to address the concerns of microbial contamination
given their lower margins of safety. Despite uncertain- if finished water is protected from recontamination
ties inherent in the analysis, these results demonstrate during storage and transport and if multiple barriers
that following proper diligence and employing tailored and quality assurance strategies are in place to ensure
advanced treatment trains and/or natural engineered reliability of the treatment processes (see Chapter 5).
treatment, potable reuse systems can provide protec- The committee’s analysis is presented as an exemplar
tion from trace organic contaminants comparable to (see Appendix A for details and assumptions made)
what the public experiences in many drinking water and should not be used to endorse certain treatment
supplies today. schemes or determine the risk at any particular site
With respect to pathogens, although there is a without site-specific analyses.